This invention relates to the formation of metal layers on substrates by non-isothermal, or non-equilibrium, plasma treatment.
The deposition of metal coatings onto solid substrates forms the basis of many everyday applications; these include: decorative finishings, electronic circuit components, gas barrier layers, gas sensors, and gas separation membranes. Methods currently employed for their fabrication include: chemical vapour deposition (CVD), electroplating, reduction of supported salts by laser, electron or ion beams, sputter deposition, electroless plating, physical vapour deposition, retroplating, thermal treatment of polymer supported metal salts, and metal hydride reduction. All of these methods suffer from at least one of the following drawbacks: copious solvent use, high temperatures, expensive vacuum apparatus, or exotic metal precursors.
An alternative approach is provided by the present invention, which is based on the non-equilibrium plasma treatment of supported metal precursors. The invention provides a method for the production of a metal film on a solid substrate which involves coating a substrate surface with a metal precursor and reducing said metal precursor by means of non-equilibrium plasma treatment. The metal precursor is coated from a solution via spin coating or dipping or solvent casting or spraying onto a substrate (or pre-treated substrate) and then treated with a non-isothermal (non-equilibrium) plasma to form a metal film, said treatment effectively reducing the metal precursor to the corresponding metal.
Metal precursors which are suitable for use in accordance with the method of the present invention include organometallic compounds, metallorganic compounds and salts of suitable metals. A wide range of metals may be applied to substrate surfaces using the method of the present invention, and particularly favourable results have been achieved using precursors including, for example, the acetates, nitrates and chlorides of palladium, platinum, gold and silver.
Various plasmas are available for use in the method of the invention, and these include non-equilibrium plasmas such as those generated by radio-frequencies (RF), microwaves or direct current (DC). They may operate from above atmospheric to sub-atmospheric pressures according to the known state of the art. Typical plasmas include low pressure RF plasmas, low pressure microwave plasmas, atmospheric microwave plasmas, atmospheric silent discharge plasmas and atmospheric glow discharge plasmas.
The plasma treatment is advantageously carried out in the presence of a feed gas to provide improved flow. Examples of suitable feed gases are hydrogen and the noble gasesxe2x80x94helium, neon, argon, krypton and xenon.
Any suitable substrate may be used when performing the invention, among the most useful being metals such as aluminium, polymers including nylon 66 and polytetrafluoroethylene (PTFE), and glass. Furthermore, the shape and form of the substrate is not limited so that, for example, containers of various styles and dimensions may be treated by the method of the invention, in addition to planar substrates.
If the substrate which is employed is non-wettable towards the metal precursor solution (e.g. a PTFE substrate) then a plasma polymer coupling layer (e.g. maleic anhydride, allylamine, acrylic acid, etc.) can first be deposited to improve the adhesion of the metal precursor to the substrate. The metal precursor can then be deposited onto this plasma polymer layer and subsequently reduced. Optionally, the metal precursor may be dissolved in solution with a suitable polymer and coated on the substrate together with the said polymer.
Improved adhesion may also be achieved by subjecting the supported metal precursor to an oxidising plasma pre-treatment step prior to the non-equilibrium plasma treatment. The oxidising plasma pre-treatment is carried out in the presence of oxygen as the feed gas.
Various coating solvents are useful for coating the metal precursor, as would be apparent to those skilled in the art, the principal criterion in selection being the solubility of the precursor in the solvent. Thus, many common organic solvents, in addition to aqueous media, provide suitable coating solvents. However, particularly favourable results have been achieved when using chloroform or, most preferably, acetonitrile as the coating solvent. Coating efficiency may be enhanced by the incorporation of a surfactant in the coating solution, preferably a non-ionic surfactant, most preferably a non-ionic alkyl phenol ethoxylate such as Triton(copyright) X-100. In this way, the adsorption of the metal precursor on to the substrate can be increased, leading to increased adhesion of the plasma-reduced metal.
Of particular interest is the use of a metal, preferably aluminium, as the substrate. Most advantageously, a substrate comprising aluminium which has been grained and anodised on at least one surface may be used to facilitate the production of a lithographic printing plate precursor. Preferably, in this case, the deposited metal is silver, which may be conveniently deposited from a solution of a silver salt such as, for example, silver nitrate. The improved adhesion associated with the use of a surfactant in the coating solution is especially beneficial in such cases, providing enhanced print endurance during printing operations on a printing press. Lithographic printing plate precursors provided according to the method of the present invention may be directly imaged by means of ablative techniques, for example imagewise thermal exposures, prior to mounting on a printing press. The advantages in terms of time and expense of such techniques, which avoid the necessity for the use of costly intermediate film and processing chemicals, are well known to those skilled in the art.
It is known that an ablative printing plate may be produced by forming silver on to a grained and anodised aluminium substrate and imagewise exposing such a precursor to a high powered laser, preferably one outputting at infra-red wavelengths. Such precursors can be manufactured by the electroless deposition of a silver salt, or through the photographic diffusion transfer process, as described, for example, in PCT patent applications nos. EP 98/03474, EP 98/03475, EP 98/03476, EP 98/03480, EP 98/03481, EP 98/03482, EP 98/03483 and EP 98/03484. However, the manufacture of such precursors is both complex and expensive. The method of the present invention, on the other hand, provides a cost effective route to the manufacture of such a precursor. Also, unlike other methods of metal deposition used to make ablative printing plates, such as sputtering or vacuum deposition as described in Japanese patent application no. 37104/1977, the method of the present invention is capable of producing silver in a more finely divided colloidal form which absorbs infra-red radiation more efficiently and thus gives rise to increased sensitivity.